Classes in different units can share the same namespace.

For example, the two units below are correct:
Unit One: Using System;
Using System. Data;
Using System. configuration;
Using System. Web;
Using System. Web. Security;
Using System. Web. UI;
Using System. Web. UI. webcontrols;
Using System. Web. UI. webcontrols. webparts;
Using System. Web. UI. htmlcontrols;
Using Mynamespace;

Namespace Mynamespace
{
Public   Class Subclass: baseclass
{< br> Public subclass ()
{< br> /// todo
}
}
}

Unit 2: Using System;
Using System. Data;
Using System. configuration;
Using System. Web;
Using System. Web. Security;
Using System. Web. UI;
Using System. Web. UI. webcontrols;
Using System. Web. UI. webcontrols. webparts;
Using System. Web. UI. htmlcontrols;

Namespace Mynamespace
{
Public   Class Baseclass
{< br> Public baseclass ()
{< br> /// todo
}
}
}

It is allowed in C.